Furthermore, the elucidation of chiral structures in the context of the origin of biomolecular asymmetry is influenced and flanked by techniques such as magnetic optical rotation dispersion, magnetic and vibrational circular dichroism, linear dichroism, but also Raman optical activity. These techniques will be briefly introduced; interested readers will be referred to the literature.
Substances that are optically active in the absence of external influences are said to exhibit "natural" optical activity. Otherwise, all substances in magnetic fields are optically active, and electric fields can sometimes induce optical activity in special situations (Barron 2004). In a magnetic field, a helical character can be induced into a linear electron displacement in achiral molecules. Here, magnetic optical rotation dispersion (MORD) and magnetic circular dichroism (MCD) spectra are obtained even for achiral molecules. The observed values are biggest if the light is parallel to the magnetic field. Thiemann and Jarzak (1981) already described the use of and explained this technique in the context of research on the origin of biomolecular asymmetry. The theoretical fundament of which can be found in Barron (2004).
Conventionally, and as we have discussed above, optical activity phenomena have been related almost entirely with electronic transitions in chiral molecules. More recently, optical activity measurements have been extended to different vi-brational states in chiral molecules making vibrational circular dichroism (vCD) spectroscopy more and more popular. As given in Eq. 2.16, vCD is the differential absorption (AA) of left- and right-circularly polarized infrared light by vibrating molecules (Berova et al. 2000).
Here, CD signals can be obtained in the infrared region of the electromagnetic spectrum. vCD techniques are particularly useful for the study of the conformational characteristics of proteins and nucleic acids but also smaller molecules like chiral pharmaceuticals. vCD spectroscopy can be used together with ab-initio calculations to determine the absolute configuration of chiral organic molecules.
Switching from the low-energetic range in vCD spectroscopy to the high-energetic spectral range, one can perform vacuum-ultraviolet (VUV) circular dichroism spectroscopy. VUV-CD spectroscopy attracted a certain interest in the study for the origin of life, which we will discuss in detail in Chap. 6. Synchrotron light can be used to measure CD spectra down to approx. 100 nm (Meierhenrich et al. 2005b).
Besides vCD and VUV-CD techniques, linear dichroism (LD) spectroscopy is applied to the study of an anisotropic arrangement of molecules that shows light absorption according to their orientation in space (see Rodger and Norden 1997). Here, vertical linearly-polarized light is absorbed by vertically oriented molecules (A|), such as oriented polyaromatic cyclic hydrocarbons, in a different way than horizontally linearly-polarized light (A^). A non-oriented collection of molecules in solution is inactive in linear dichroism. A well-defined orientation of molecules for example in crystals, films, or on structured surfaces is required for LD spectroscopy. LD signals correspond to the differential absorption of oriented linearly polarized light as indicated in Eq. 2.17.
A specific kind of spectroscopy called Raman spectroscopy can be used as supplementary tool to gain information on the chirality of organic molecules. This spectroscopy uses the Raman effect, which was observed by the Indian physicist and Nobel price winner Sir Chandrasekhara Venkata Raman. The Raman effect is based on photon scattering on the surface of matter. The spectrum of organic compounds, liquids, or solids irradiated with monochromatic light does not only show the Rayleigh scattering9 at frequencies of mo, but also the so-called Raman-lines
9 Rayleigh scattering is the scattering of electromagnetic radiation by particles that are much smaller than the wavelength of the electromagnetic radiation.
at frequencies of rn0 ± rn. In contrast to Rayleigh scattering, in the case of Raman scattering energy and momentum are exchanged between photons and scattering medium. Scattered light at the frequency of rn0-rn corresponds to the Stokes-line, the frequency rn0 + rn corresponds to the Anti-Stokes-line.
It is worth mentioning that if chiral molecules are used as scattering medium, the Rayleigh and Raman scattered light is to a certain degree circularly polarized and the scattered intensity is slightly different in right- and left-circularly polarized light (Atkins and Barron 1969). Raman Optical Activity (ROA)-signals indicate the difference between the scattered intensities in right- and left-circularly polarized light as written by Eq. 2.18.
This observation led to the recent development of the ROA technique, applicable to a huge range of chiral samples of central importance for life, from small organic molecules over proteins, carbohydrates, nucleic acids to intact viruses (Barron et al. 2007). The significance of ROA is that it records - such as vCD techniques - vibra-tional optical activity and therefore provides more stereochemical information than standard chiroptical techniques of visible and ultraviolet CD, measuring electronic optical activity. For many molecules conformational details are visible by ROA, whereas visible and ultraviolet CD spectroscopy provide information on the stereochemical environment of the chromophore only. Molecules lacking a chromophore are inaccessible to visible and ultraviolet CD but can be studied with ROA. Very recently, a commercially available ROA instrument was introduced, using a visible laser beam at 532 nm from a frequency-doubled Nd/YAG laser (ChiralRAMAN by BioTools Inc.).
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